Rebuild Schedule for Ammonia Valves

We have 3 Frick 700 HP compressors (R717). Recently we did some maintenance on the fanbanks and found that some of the valves were not functioning properly. How often should valves used in an Ammonia System be rebuilt? These compressors service 7 fanbanks in 2 large warehouse cooler areas (38 oF) and 6 fanbanks in a large warehouse freezer (-15 oF). We noticed during a hot gas defrost in the freezer that one fanbank was heating up two other fanbanks.

The valves were installed without limit switches and there is not enough room for flow transducers (delta P), has anyone had experience with the new doppler sensors?

Thanks,
Amber

I think that fanbanks are evaporators. Am I right? Usually, valves should be replaced if they leak through. I didn't get about limit switches, flow transdusers and doppler sensors? Why do you need them?

The valves allow ammonia to flow through the cooling coils of the fanbanks. I use limit switches to determine if a valve has actually opened after I send the command from the PLC. Otherwise, how can I tell if the valve I commanded to open has actually opened? For the coolers, there are two valves - one with cooling liquid and the other the suction side going back to the compressor.

There are many ways to make sure that liquid solenoid is open. Evaporator(fanbank) will be frosted or sweated. If liquid solenoid is closed, temperature in refrigerated room will go up.